Coil Over Plug Ignition Systems

There were many types of ignition systems used throughout the years and things are continuing to evolve. A coil over plug "COP" ignition eliminates the distributor and secondary wires and replaces them with the PCM, ignition module, and a coil mounted directly over the plug of each cylinder. This electronically controlled ignition system is very efficient.

In order to properly diagnose a misfire on one or all of the engine's cylinders, it's important to understand the system you're working with. Let's have a look at the components that make up a typical coil over plug ignition system.

Coil on Plug Ignition System

COP coils: The coil shown in the illustration is typical and integrates the ignition module into the coil itself. The primary and the secondary circuits are located in the coil. It's where the two meet. If there is no power received by the ECM, neither circuit gets power and the coil will not fire. Notice the trigger pin, it's a member of the control circuit. With some COP systems the trigger circuit also contains a feedback circuit performing a separate function. This feedback circuit is used by the PCM to confirm that the cylinder fired. In this type of system the ignition module is located inside the PCM.

Related wiring: Make sure the connectors, wires and harness are in proper operating condition with a good visual inspection. Sometimes the connectors have locks that dry out and can easily become damaged during removal and will not snap back into place correctly after removal. Perform a wiggle test and check these connectors by pulling lightly on the plug to make sure it correctly seated into it's companion socket. Connectors contain a mechanical snap and a rubber seal to prevent environmental exposure. Check any suspected wiring for resistance with an ohmmeter and compare with manufacturer's specifications. Loose connectors result in an intermittent condition.

Ignition Control Module: The ignition control module takes place of the distributor in a distributor-less (EI) ignition system. It receives it's timing instructions from the PCM based on input from the crankshaft and camshaft sensors and sends the appropriate signal to the appropriate cylinder. The IM may be stand alone, located in the ECM or integral to each ignition coil like the coil used in the illustration.

Engine Control Module: The PCM may or may not contain the ignition control module. It takes input from sensors like the crankshaft and camshaft sensors and calculates ignition timing by comparing it to preset instructions. It then sends this signal to each coil in the correct firing order. An open between the PCM and a cylinder's coil or a faulty PCM driver will result in no power and a misfire on that cylinder.